Traditional semiconductors and IC devices are typically packaged in a variety of ways to provide redistribution from the terminals on the die to a spacing that is conducive to cost effective printed circuit board (“PCB”) fabrication techniques. The packages also serve to protect the fragile silicon or provide additional functions such as thermal management or near device decoupling. In many cases, the size and distance between die terminals is so small that the IC device cannot be connected to the final PCB without some sort of re-routing interface.
Historically, pin grid array packages (“PGA”) were used for CPU devices. A socket was soldered to the motherboard and the field of pins extending out of the package was plugged into the socket. One advantage of a PGA is that the IC device can be removed from the socket if needed. The pin on PGA devices degrade signal performance. The pins are also expensive and are subject to damage during handling.
Land grid array (“LGA”) packages are now used for many desktop and server applications, and ball grid array (“BGA”) packages for direct soldering in notebook computers.
There are many types of connectors used to mate one circuit board to another. A LGA package acts essentially as one PCB and the mother board the other. These connections are made vertical in what is often called a mezzanine style. There are also various right angle connectors called backplane connectors. These consist of a male half and female half that are plugged together to make the connection. In relative terms, however, the size of the contacts used in the connectors is very large compared to those used in the LGA type CPU sockets.
One advantage of PGA devices is that the socket mechanically engages with the pin to secure the PGA package to the PCB. With LGA devices, on the other hand, a mechanism is required to apply a clamping pressure to hold the device against the socket contacts. These clamping mechanisms apply a fairly significant load to the system, and can cause circuit board warpage, broken solder joints, or flexing of the package. In many cases, a heat sink and fan assembly is added to the system by passing fasteners through the circuit board and mating with a stiffening member on the bottom side of the board.
Next generation LGA devices create many challenges due to the increased performance requirements, multi-core processors which drive the terminal count up, and low voltage differential signal architectures which can challenge terminal routing density. The IC device is much smaller than the associated package due to the need to fan out the pitch of the terminals so that the motherboard can be made cost effectively using standard four-layer technology. There is a need to shrink the pitch of the terminals so the package size can be reduced, as is done in cell phones and CPU sockets. The challenge is that it is very difficult to make a socket with contacts on a finer pitch that have enough compliance to accommodate the warpage of the package and provide stable contact resistance. It is also very difficult to route the pattern at the fine pitch and keep the PCB at four layers, unless a very expensive blind and buried via method is used.